Early in the story, Roger is described as being reclusive shy and quiet. Roger repeatedly bullies the childern by ruining their sandcastles, throwing sand in their eyes, and throwing rocks at them. Primarily, Roger throws the rocks to miss, with "the protection of parents and school and policemen and the law. Roger's arm was conditioned by a civilization that knew nothing of him and was in ruins." The taboo of civilization clung around him and prevents him from actually hurting the other boys.
During the second successful pig hunt, Roger is the most agressive. After killing one of the sow's various piglets, he continues to brutally torture her. Once he finds a lodgment for his spear, Roger shoves it right up the pig's posterior. Afterwards, he sharpens a stick at both ends on Jack's command and helps in sticking the pig's head on the spear, as an offering to the Beast.
After another boy named Simon discovers the truth about the Beast, he stumbles into the circle of savages while they are doing a reenactment of the hunt on the beach, where he is bloodlessly stabbed to death. Neither Maurice nor Jack feel any remorse for their actions. The day after that, on top of Castle Rock, Robert tells Roger that Jack is going to beat Wilfred. Roger hopes to torture Wilfred and hurries towards the rest of the tribe to do so. Even though he doesn't get the chance to hurt Wilfred, he evolves to the status as Jack's 'second in command' - the chief torturer of the boys.
From Roger's vantage point above when Piggy was making a speech at Castle Rock, "Ralph was a shock of hair and Piggy a bag of fat." That dehumanization allowed him to become completely free of the restraints of civilization and fully give into his sadistic urges. "High overhead, with a sense of delirious abandonment, Roger leaned with all his weight on the lever." He pushes a boulder off the cliff and onto Piggy, killing him. After that, while Samneric are captured, Roger hurts them and forces them to join Jack's tribe. And before they start the hunt for Ralph, he hurts them again and makes them tell where Ralph is hiding.
The night before the hunt for Ralph, Roger sharpens a stick at both ends(completely on his own will), when Jack plans on hunting and killing Ralph. Roger plans on cutting off Ralph's head and impaling it on the stick as another offering for the Beast. Roger and Jack's dark intentions are only stopped at the sight of the naval officer. There, Roger's sadistic pleasures are reigned back as reality sets into them all.
Roger is a sadist and represents bloodlust and evil at its worst. He revels in inflicting pain on others and is only restrained by the rules of society.